Method of treating acid solutions



T. c. OLIVER er Al. 2,021,867

METHOD oF TREATING om sonmlcmsl n' Nw. 19, 1935.

'iled may 1o. 1953 Mama/P470? yc @Nuff/v0? r cbm/irak' 3 2 INVENToRs71?@745 C. O /yE/P,

' ATTORNEY.

hiPatented'Nov. 19, 1,935`

METHOD OF TREATING ACID lSOLUTIONS Thomas C. Oliver,- Great Neck, andSamuel F.

Spangler, Scarsdale, N. Y., assignors to ChemicalConstructionCorporation, New York, N. Y., a corporation of DelawareApplication May 10, 1933, Serial No. 670,272 lIn Great Britain January28, 1933 'I Claims. (Cl. 23-177) This invention relates to the treatmento'f solutions containing salts and the free acids thereof, for thepurpose of recovering -the acid radical therein. More particularly, thepresent inven.

tion is concerned with the treatment of the waste solutions produced inthe steel industry from acid solutions used for the cleaning of ferrousmetals, commonly known as pickling solutions.

The spent pickling solutions usually contain fairly large amounts offerrous and/or ferric sulphate and a small amount of free sulphuric acidvand often the content of these substances is relatively high.Ordinarily the spent liquors contain 'perhaps "1% of HzSr and up to 20%of iron sulvaluable free acidhthese solutions cannotbe fur- ,ther usedexcept with difficulty and at great ex'- pense.- It is, therefore,highlydesirable to provide a process for 'recovering the valuablesulphate radical in such a form as to make it usable and to thisend'there have been proposed a number of processes whereby the sulphateradical is 'recovered by first mixing the iron salt with a reducingagent and then heating until a reaction takes place liberatingsubstantially all of `the sulphur content in the form of SO2 which isthen oxidized to S03 and thereby converted into ,sub-

stantially vpuregsulphuriczacid, which may now,

again be utilized.

The present invention `is designed as an improvement in such processwhereby 4certain advantages of operation are obtained.' Itmakes itpossible to recover fromspent acid solutions not only the acid fixed as'sulphate but also the free acid-in a' single simple operation.Previously there wasused a cumbersome wasteful and ex'- pensive methodwhereby the solution was partially evaporated and cooled to crystallizesome sulphate and the cycle was repeated. It never ,produced a pure acidand the crystallized sulphate was contaminated with acid causinga'nuisance besides a loss of acid. Y j

The basis of the (present invention is lthe use of an oxide toneutralize .the free acid and thereby facilitate the succeeding steps ofthe process.

sulphuric acid, oxide of iron is used and preferably this oxide is inthe dust form and preferably derived from a later stage of the process,in

'order that the reaction thereof with the acid may be facilitated. Inthe later steps of the process wherein the residueof iron oxide isroasted to give the nal product, a certain amount Vof' light, line ironoxide dust, which has been considered undesirable, is formed and this iscollected in a 5 y.

well known dust collecting system. For the prest ent invention this dustlis highly desirable assit combines large volume with large surface areafor ythe action of the acid, whereby the reaction to form ferroussulphate, for example, isgreatly facilitated. v f

We` usually provide a substantial excess over the theoretical amount ofoxide necessary to neutralize the free acidand such excess may be from40% to 110% over the theoretical amount and l5 even more. An excess ofoxide has no detrimental effect in the later steps of the process and ithas the great advantage of causing the neutralizingreaction to go tocompletion in a short time. An excess of about 50%,has been found 20generally satisfactory.' Furthermore, the dust 'added to this stage ofthe process will to a great extent become sintered at the end of theprocess and thus be converted into a more desirable form.-

The oxide preferably used in this process is specifically an oxide ofJiron and this is generally a mixture of the various-well-known oxides,such as FeO, FezOs, Fea04 depending upon the lconditions in the finall,sinterng operation. Usually it is principally F6304 with a. minor- 30amount of FeO, and a little FezOs. Whatever the composition the finalresult is the same, namely, it neutralizes the free sulphuric acid.

We have found that under ordinary condi- -tions iron oxide does notreact with sulphuric 35.

acid to neutralize the same. We have found that a reaction may be.caused to occur between ironv oxideand sulphuric acid only at certainstrengths of acid and at certain temperatures. We have found that it isnecessary to heat the solution to above the boilingl p'ointof water andpreferably to atemperature of about230 to 235 F. and up to about 250 F,and to have the acid .at `a strength of about 30 to 40 per cent H2S04.Under such conditions the reaction'of the acid with theA 45 oxide takesplace with a fair degree of rapidity and it goes readily to completion.

We have further found that the presence of iron sulphate in thepicklinglsolution has a -distinct effect on the reaction. which takesplace bey In actual practice with spent pickling solutions of/f'tweenthe iron oxide andthe free acid. A solution containing a definite amountof iron sulphateeand to which there is added a suiiicient amount'of ironoxide to neutralize all the free acid gave no reaction at the boilingtemperature '65 cent of free acid. When the acid concentration at theboiling point was 30 per cent some reaction was apparent. When theconcentration was 35% the reaction was more noticeable and in a 40% acidconcentration the reaction was rapid.

On the other hand when a mixture of sulphuric acid solution with ironoxide was made in the absence of iron'sulphate quite diiferent resultswere obtained. Mixtures at the boiling point containing from 20 per centup to 50 per cent of sulphuric acid gave'no reaction with iron oxide.When the acid'concentration was 55 per cent at theboiling temperaturesome slight action was noticeable and at per cent concentration theaction was`vigorous. From theseuresults we believe that iron sulphate isof material assistance in promoting the reaction between free sulphuricacid and iron oxide.

In the-accompanying drawing constituting a part hereof the single figureis a flow-sheet showing diagrammatically an apparatuswhich is adapted tocarry out the present invention.

There is provided a slightly inclined cylindrical drum kiln ordehydratorI having vbaiiles 2 and 3' for the purpose of dehydrating a solution 4which is contained in a vessel 5 from which a pipe G leads intp theinlet ed 'I of the dehydrator.

Finely divided iron oxide dust 8 contained in a hopper 8 is fed into thestream of liquid by pipe I0 connecting with pipe B or directly into theinlet end I of the drum I in the proper relative amounts. The materialtravels over the baflles 2 'and 3'and at the exit end II is asubstantially dry material which is removed through duct I2 and iscarried away on conveyor I3. The heat vnecessary for the dryingoperation is provided by coal contained in hopper I4, the same beingpulverized at I5 and being blown into the dehydrator through blower I6and furnace I1. In place of coal, oil or gas or other fuel may be used.'Ihe temperature of the combustion gases as they enter the end IIofvdehydrator I is generally from 18002200 F. and the temperaturethereof at the exit end 1 is approximately 350 to 450 F. The temperatureof the dried material at the exit end of the dehydrator is in theneighborhood of 600 F.

`\ 'I'he reactions taking place in the dehydrator` are briefly that atthe end 'I of the dehydrator there is a. gradual heating of the mixtureandevaporation of water until the temperature has risen sumciently highand the concentration of the acid is such that a reaction can occurbetween the iron oxide and the sulphuric acid. InI the central sectionof the dehydrator the neutralizing reaction goes substantially tocompletion with the formation of water, which is evaporatedsubstantially as fast as it\is formed. The temperature in this sectionis probably fairly constant. At the exit end II there isevaporatherethrough and is met by a countercurrent of tion of water andtoward the end of such evaporation the temperature rises until completedryness is obtained and the material leaves the dehydrator.

The dried material on the conveyor I3 can be mixed with pyrites I8 andwith coal through the duct I9 and the mixture is fed through duct 20intothe slightly inclined rotating drum roaster 2| having baiiles 22therein. The material passes gases produced by combustion of fuel fromduct 23 in furnace 24 by the blast of air from blower 25. Reactionsoccur by reason of the admixture of iron sulphate with the .carbongceousreducing when the mixtures contained from 20 to 25 per as somefurtherroxidation to take place.

agent at the inlet end 26 of the roaster 2l whereby SOz is produced andis-removed through pipe 2'I through dust collector or separator 28 andthrough exit pipe 29, which leads to an appa- .ratus for producingsulphuric acid therefrom. 'I'he material passes along to the exit end 30of the roaster where the high temperature of the combustion gases causesa sintering action as well The sintered oxide of iron which is usually amixture of various iron oxides is removed through duct 3|, conveyer 32and is stored in hopper 33.

The iron oxide 8l is generally obtained from separator or dust collector28 from the bottom ,34 of which the very finely divided dust flows intothe hopper 9 for use in the process.

'I'he temperature of the gases atl the furnace 24 is sufllciently highso as to produce a temperature at the exitend 30 of the roaster of about1400l800 F. At the inlet end 26 thereof the temperature of said gases isfrom 600 to 1000 F.

leaves the roaster is about 600 F. As a typical example of the operationof this invention one may take pounds of spent acid, :5

`Assuming that the ox'idels in the form or Feo,

the reaction is as' follows:

FeO+H2S04=Feso4+mo As vthe material passes on further reaction takesplace. First, there is the evaporation of water ferrous sulphatecontaining from about 1/2 to 1 pmol. of H2O. Because of the completeneutralization of the acid by the oxide there is no material amount Aofreduction in the dehydrator and, therefore, no loss of SO2 or SO: gasestherein.

'Ihe addition of oxide which largely corre ferrous sulphate sulphuricacid solution before concentrating, there is practically no oxidation ofthe ferrous sulphate by the, sulphuric acid, although when ferroussulphate is heated with sulphuric acid in the absence of Fe304, ferricsulphate is produced. It'is, voi! course, undesirable to produce largequantities of ferrie sulphate as thistends to give olf SO: in theroasting step which follows and it is o ne of the advantages of thepresent invention that on roasting, an SQ: gas free from SO: isproduced. 4 f" The dried mass is, after admixture with/'coal andmetallic sulphides such as D'yrites or pyrrhotite, then transferred tothe roasterkiln where the material passes through the kiln. Because ofthe presence of the reducing agent, such as carbon, a reaction asfollows will take place, assuming the ferrous sulphate is 4in the formof the mono-hydrate.

znesolmo+c=2reo+2sol+co= l I'he mixture is The temperature of the`sintered product as it :pantser From the above description of ourinvention it is apparent that 'a number of advantages are obtainedthereby. The use of iron oxide as a neutralizing medium provides a cheapmeans for obtaining the desired result. It furthermore uti-`neutralization is decreased because neutralization takesplace atelevated temperatures'. The

excess' of oxide insures the ready `and complete neutralization of theacid. By reason of the continuous movement of the charge in the roastercountercurrent to the heating gases, the reduction takes place uniformlyand at a constant rate, resulting iny a substantially constantconcentration Aof SO2 in the gases produced in the operation.

Although we have described our invention lsetting forth a specificembodiment thereof, oui` invention is not limited to the'details setforth.

For example, Aother oxides than those produced at the end of the processmay be used andthe oxide need not be in finely divided, dust form. Thetemperatures, times of treatment, concentrations and proportions ofvingredients may `be varied to suit conditions.. T'he process is notlimited to the treatment of spent pickling solutions, but othersolutions of similar character may also be treated thereby. Theinvention is further 'applicable to solutions of other substanceswherein there is free acid and'crystallizable salts which tend tointerfere with the recovery of the acid radical. In the claims referenceto pickling solutions and the sulphate radical is intended tocover theequivalents thereof as set forth herein. The iron oxide may be any ofthe well known oxides or mixtures thereof. The salts in the picklingsolution may be ferrous or ferric sulphates or mixtures thereof andeither or both of these sulphates may be formed in the neutralizingstep. In the dehydrator and roaster there is provided steel rails forbreaking up incrustations as they may form, but they may be omitted orother means substituted therefor. Our invention is to be broadlyconstrued and is not to be limited except by the termsof the claimsappended hereto.

1. A method o f treating solutions containing free sulphuric acid andferrous sulphate which comprises mixing therewith an iron oxide con- 5taining as a major componentoxide corresponding to the compositionFe304, heating the mixture to a sumciently high temperature to causecombination of said oxide with said acid, evaporating the mixturesubstantially to dryness and then heating the mixture in the presence ofa reducing agent to remove the sulphur. content as SO2 and to produce aniron oxide residue.

2. A method of treating solutions containing free sulphuric acid andferrous sulphate which comprises mixing therewith an oxide of ironproduced by the process and corresponding to the compositionFesOaheating the mixture to a sufflciently high temperature to cause thecombinationofsaid oxide with said acid, evaporating the 20 mixturesubstantially to dryness and then heating `the mixture in -the presenceof a reducing agent to remove the sulphur content as SO2 and to producean iron oxide residue corresponding substantially to the compositionFezOr, and utilizing 25 a portion of said residue to neutralize the freesulphuric acid of a further portion o f fresh solution.

3. A method according to claim 1 in which the solution is a waste liquorfrom the pickling. of 30 iron and steel.

4. A method according to claim 2 in which the solution is a waste liquorfromA the pickling of iron and steel.

5. A method according to claim 1 in which the l85 temperature to whichthe solution and iron oxidel is heated is at least 23o-240 F.

6. A method according to claim l in which the temperature to which thesolution and iron oxide is heated is at least ZBO-240 F. and thestrength 40 of the sulphuric acid vis about 30-40%.

7. A method according to claim 1l in which the amount of iron oxide isin excess of the theoretical amount vrequired for neutralizing the freersulphuric acid.

THOMAS C. f OLIVER. SAMUEL F. SPANGIER.

